Calculate Your Required Battery Size

Use this Lithium Battery Size Calculator to find the correct battery capacity for your application. Enter your electrical loads using the appliance table below, or type a known daily energy use in watt-hours. The calculator then applies your selected system voltage, days of autonomy, depth of discharge, and inverter efficiency to output the minimum required battery capacity in both Ah and kWh, plus a recommended pack configuration based on the battery unit size you choose.

Correct battery sizing avoids two common and costly mistakes: under-sizing, which causes frequent deep discharges that shorten battery life, and over-sizing, which wastes money on capacity that never gets used. The goal is to find the smallest battery bank that meets your autonomy and discharge depth requirements with a reasonable safety margin.

Lithium Battery Size Calculator

Step 1 — Enter Your Loads (or skip to Step 2)
Appliance / Load Watts (W) Hours/Day Daily Wh
150 Wh
1080 Wh
60 Wh
Total Daily Load 1,290 Wh/day
— OR enter daily energy use directly —
Step 2 — System Parameters
Leave blank to use the appliance table total above
Higher voltage reduces current and cable size for large systems
Number of days battery must run without recharging
LiFePO4 typically allows 80–90% DoD safely
AC loads lose energy in the inverter conversion
Select the Ah rating of individual battery units you plan to use

Why Proper Battery Sizing Matters

Avoid Costly Under-Sizing

A battery bank that is too small gets deep-discharged regularly, which accelerates capacity loss and shortens cycle life. LiFePO4 batteries designed for 3000 cycles at 80% DoD may only last 1000 cycles if routinely discharged to 100% DoD. Correct sizing protects your battery investment.

Avoid Wasted Over-Sizing

A battery bank that is much larger than needed costs more upfront and may never fully cycle, which can actually reduce efficiency and BMS calibration accuracy. Proper sizing balances upfront cost with the autonomy and longevity you need for your specific application.

Plan Autonomy for Your Application

Different applications require different autonomy buffers. An RV on a campground may need only one to two days of storage before it recharges. An off-grid cabin may need three to five days to survive extended cloudy periods. This calculator lets you set the exact days of autonomy for your situation.

Choose the Right System Voltage

System voltage affects cable size, current levels, and efficiency. A 48V system running the same load as a 12V system carries one-quarter of the current, allowing smaller cable cross-sections and lower resistive losses. Higher voltage is especially important for systems above 1000Wh/day or with long cable runs.

Calculate Series and Parallel Configuration

Once you know total required Ah and system voltage, you need to know how many battery units to connect in parallel (to add Ah capacity) and how many to connect in series (to achieve system voltage). This calculator shows the recommended configuration based on your chosen battery unit size.

Account for Real-World Efficiency Losses

Inverters, wiring, and battery internal resistance all reduce the effective energy available from a battery bank. Ignoring these losses leads to a bank that runs out of energy before the calculated runtime. This calculator includes inverter efficiency and depth of discharge in the sizing formula for more realistic results.

Battery Sizing Formula

The battery sizing formula used in this calculator follows standard off-grid and solar storage design practice:

Required Wh = (Daily Wh × Autonomy Days) ÷ (DoD × Inverter Efficiency)

Required Ah = Required Wh ÷ System Voltage

For example, a daily load of 1500 Wh, two days of autonomy, 80% DoD, and 95% inverter efficiency requires: (1500 × 2) ÷ (0.80 × 0.95) = 3000 ÷ 0.76 = 3947 Wh nominal. At 48V, this is 3947 ÷ 48 = 82 Ah. Using 100Ah batteries, one unit at 48V would meet this requirement, or two 100Ah 12V batteries in series to form a 24V bank, or four in series for a 48V bank.

The depth of discharge factor accounts for the fact that not all rated capacity should be used in normal operation. LiFePO4 batteries are typically sized for 80% DoD in off-grid applications to preserve cycle life. The inverter efficiency factor accounts for energy lost converting DC battery voltage to AC output. For DC-only systems with no inverter, this factor is 1.0.

Battery Sizing by Application

Common battery sizing guidelines vary significantly by application. The table below shows typical system voltage, capacity ranges, and recommended chemistry for the most common use cases. These figures are starting points — actual sizing depends on your specific loads, usage patterns, and autonomy requirements.

Application Typical Voltage Typical Ah Range Typical kWh Range Recommended Chemistry
Small RV / Campervan12V100–200 Ah1.2–2.4 kWhLiFePO4
Large RV / Motorhome12V or 24V200–400 Ah2.4–9.6 kWhLiFePO4
Marine / Sailboat12V or 24V100–300 Ah1.2–7.2 kWhLiFePO4
Off-Grid Cabin48V200–600 Ah9.6–28.8 kWhLiFePO4
Home Solar Storage48V100–400 Ah4.8–19.2 kWhLiFePO4
Golf Cart48V100–200 Ah4.8–9.6 kWhLiFePO4
Backup Power (Home)48V50–200 Ah2.4–9.6 kWhLiFePO4
Telecom / Industrial48V200–1000 Ah9.6–48 kWhLiFePO4
Van / Skoolie Conversion12V or 24V200–400 Ah2.4–9.6 kWhLiFePO4
Tiny Home / Container Home48V100–300 Ah4.8–14.4 kWhLiFePO4

Choosing the Right System Voltage

System voltage is one of the most important decisions in battery system design. The most common options for lithium battery systems are 12V, 24V, and 48V. For small systems with modest daily energy use — typically below 500 Wh/day — a 12V system is practical and uses widely available components. For medium systems between 500 Wh and 2000 Wh/day, 24V reduces current by half compared to 12V, allowing thinner cables and less heat generation in wiring.

For systems above 2000 Wh/day, 48V is strongly recommended. At 48V, current is one-quarter of an equivalent 12V system running the same load. This allows smaller wire gauges, reduces voltage drop over long cable runs, and supports high-power inverters without extreme current demands. Most residential solar storage systems, off-grid cabins, and home backup systems use 48V for these reasons. LiFePO4 battery packs at 48V (16 cells in series, nominal 51.2V) are widely available and well-supported.

Changing system voltage after installation is expensive — it often requires replacing the battery bank, inverter, charge controller, and wiring. Choose the voltage that supports your expected maximum load and future expansion before buying any components. When in doubt, choose 48V for systems that may grow over time.

LiFePO4 Battery Sizing for RV

RV battery sizing starts with listing all 12V and AC appliances used during a typical overnight or off-grid stay. Common RV loads include LED lighting (10–50W), a 12V compressor refrigerator (30–60W average), a water pump (40–80W burst), USB charging (10–30W), a diesel heater fan (10–30W), and occasional 120V AC loads through an inverter such as a microwave, laptop, or coffee maker. Adding these up for their typical daily runtime gives you the daily watt-hour total.

Most RVers targeting two-day solar autonomy with 80% DoD end up needing 100–200 Ah at 12V LiFePO4. A full-time liveaboard RVer or a motorhome with heavier AC loads may need 200–400 Ah. Two 100Ah LiFePO4 batteries in parallel at 12V is a common and practical starting point for weekend and part-time RV use. For full-time use with higher loads, four 100Ah batteries in parallel or two 200Ah batteries provides more comfortable headroom.

RV battery sizing also needs to account for charge sources. If solar panels are the primary charge source, the panel capacity must match or exceed daily consumption on average. If shore power is the primary source, sizing can be more conservative since the battery is regularly recharged. Always size for the worst-case scenario — maximum load with the minimum expected charge input — to ensure the battery lasts through the intended usage period.

LiFePO4 Battery Sizing for Off-Grid Solar

Off-grid solar battery sizing requires matching storage capacity to solar generation, daily consumption, and autonomy requirements. The standard approach is to size for two to three days of autonomy without solar recharge, which covers extended cloudy periods common in most climates. In regions with reliable daily sun, one to two days of storage may be sufficient. In high-latitude or cloudy regions, three to five days is more appropriate.

48V systems are nearly universal for off-grid solar storage above 2 kWh. A typical small off-grid cabin might need 3–6 kWh of battery storage, equivalent to 60–125 Ah at 48V. A larger home with air conditioning and electric cooking may need 15–30 kWh of storage. LiFePO4 is the preferred chemistry for off-grid solar due to its long cycle life (3000–6000 cycles), wide operating temperature range, and safety profile. The long cycle life is especially important for off-grid systems where the battery cycles daily for years.

Solar panel sizing for off-grid systems is a parallel calculation. A common guideline is to install enough solar to recharge the battery bank in one to two sunny days. For a 10 kWh battery bank, this means 2–4 kW of solar panels in a good sun location. Always account for panel orientation, shading, temperature derating, and charge controller efficiency when sizing the solar array alongside the battery bank.

Series vs Parallel Battery Connections

Connecting batteries in series increases the total bank voltage while keeping Ah capacity the same. Two 12V 100Ah batteries in series create a 24V 100Ah bank. Four 12V 100Ah batteries in series create a 48V 100Ah bank. Series connections are used to match the required system voltage from individual battery units that have a lower nominal voltage than the target system voltage.

Connecting batteries in parallel increases the total Ah capacity while keeping voltage the same. Two 12V 100Ah batteries in parallel create a 12V 200Ah bank. Four 12V 100Ah batteries in parallel create a 12V 400Ah bank. Parallel connections are used to add capacity at the same voltage. Combining series and parallel — called a series-parallel configuration — achieves both higher voltage and higher capacity simultaneously.

When designing a battery bank, first determine the required system voltage (which determines how many batteries are needed in series), then determine the required Ah capacity (which determines how many series strings are needed in parallel). The total number of batteries is the number in series multiplied by the number of parallel strings. Always use identical batteries — same age, chemistry, capacity, and ideally the same manufacturer batch — when building parallel banks to avoid current imbalance and uneven aging between units.

Common Sizing Mistakes to Avoid

  • Sizing based on nameplate Ah without accounting for DoD: A 100Ah battery with 80% DoD provides only 80Ah of usable capacity. Always calculate required Ah based on usable capacity, not total rated capacity.
  • Forgetting inverter efficiency losses: AC loads consume more energy from the battery than the appliance wattage implies. A 1000W AC load running through a 90% efficient inverter draws 1111W from the battery. Not accounting for this understates required battery size.
  • Using peak load current instead of average consumption: Appliances cycle on and off and rarely run at full power continuously. Use average power draw and actual daily runtime rather than peak wattage for sizing calculations.
  • Choosing 12V for a large system to save money on the inverter: A larger inverter at 12V carries very high currents that require thick, expensive cables and creates significant voltage drop. The cable cost savings from choosing 48V over 12V often exceed the price difference in inverters for systems above 2 kWh.
  • Not leaving headroom for future loads: Many system owners add loads after installation. Sizing the battery bank with 20–30% extra capacity over the current calculated requirement makes it easier to add appliances without replacing the entire battery bank.
  • Mixing old and new batteries in a parallel bank: Adding new batteries to an existing parallel bank can cause current imbalance. New batteries with higher capacity and different internal resistance will share load unevenly with aged batteries, potentially stressing both. Replace the full bank when expanding if possible.

Related Tools

Use these related calculators to continue your battery system design and sizing workflow:

Frequently Asked Questions

What size lithium battery do I need for an RV?
A typical RV needs 100–200 Ah at 12V LiFePO4 for basic use with one to two days of autonomy. A full-time liveaboard RV with heavy loads may need 300–400 Ah. Start by adding up your daily watt-hour consumption, divide by system voltage, account for 80% DoD, and multiply by your desired days of autonomy to find the minimum required Ah.
What size battery do I need for off-grid solar?
Off-grid solar systems are typically sized for two to three days of autonomy at 80% DoD. Use a 48V system for efficiency. A small cabin using 2 kWh/day with three days of autonomy at 80% DoD needs (2000 × 3) ÷ 0.80 = 7500 Wh, or about 156 Ah at 48V. LiFePO4 is strongly recommended for off-grid solar due to its long cycle life.
Should I use 12V, 24V, or 48V for my battery system?
Use 12V for small systems below 500 Wh/day with short cable runs. Use 24V for medium systems between 500 and 2000 Wh/day. Use 48V for larger systems above 2000 Wh/day, long cable runs, or high-power inverters. Higher voltage reduces current, allowing smaller cables and less resistive loss. Once installed, changing voltage is expensive, so choose a higher voltage if future expansion is likely.
How do I calculate battery size from daily watt-hours?
Use the formula: Required Ah = (Daily Wh × Autonomy Days) ÷ (System Voltage × DoD × Inverter Efficiency). For example, 1500 Wh/day, 2 days autonomy, 48V, 80% DoD, 95% efficiency: (1500 × 2) ÷ (48 × 0.80 × 0.95) = 3000 ÷ 36.48 = 82 Ah. Round up to the nearest available battery unit size.
How many 100Ah batteries do I need?
It depends on your system voltage and required Ah capacity. For a 12V system needing 200 Ah, you need two 100Ah 12V batteries in parallel. For a 48V system needing 200 Ah, you need four 100Ah 12V batteries in series (to reach 48V) as one parallel string — or one 100Ah 48V battery. Use the calculator above to get the specific number for your load and configuration.
What is depth of discharge and why does it matter for sizing?
Depth of discharge (DoD) is the percentage of rated capacity that is actually used during a discharge cycle. A 100Ah battery discharged to 80% DoD uses 80Ah before recharging. DoD matters for sizing because you should not design a system that requires more than the recommended maximum DoD, or cycle life will be reduced. For LiFePO4, 80% DoD is standard. If you size a battery for 80 Ah of daily use at 80% DoD, you need a 100 Ah battery.
How many days of autonomy should I plan for?
For solar systems with daily sun, one to two days is often sufficient. For systems in cloudy regions or where solar recharge may be intermittent, two to three days is standard. For remote off-grid locations or critical backup systems, three to five days provides a comfortable safety margin. More autonomy means more battery cost but greater independence from recharging sources.
Can I mix different Ah batteries in a bank?
Mixing different Ah batteries in a parallel bank is strongly discouraged. Batteries with different capacities, ages, or internal resistances will share current unevenly, causing the smaller or weaker battery to be over-stressed while the larger battery is under-utilized. For best performance and longevity, always use identical batteries — same capacity, chemistry, age, and manufacturer — in all parallel connections.